1. Field of the Invention
The invention relates to Flight Management Systems for aircraft, and in particular to an apparatus and method whereby time-constrained flight can be achieved while maintaining predetermined input parameters selected for minimizing cost of flight, wherein arbitrary points in the flight plan can be designated as time-constraint points, and wherein flight segments can be arbitrarily selected for exclusion from any speed variation.
2. Description of the Prior Art
A Flight Management System (FMS) provides the crew of an airplane with extensive capabilities for planning and carrying out the flight of their aircraft. One of the principal goals of the modern FMS is to optimize the cost of the flight, based on a predetermined set of parameters. In current practice these parameters are specified so that they define the actual cost in fuel and flight time for a given flight plan. These parameters are determined carefully so that they match the performance characteristics of the aircraft under consideration as closely as possible. The result of this exercise in optimization is a flight plan that contains target altitudes and speeds for each leg or segment of the flight. However, since the objective is to provide an optimum flight plan (i.e., a minimum cost of flight), the time of arrival at any destination is a result of the optimization calculations, and therefore will not necessarily match any desired arrival time, which may be constrained by factors such as costs of crew time, need to match schedules of connecting flights, and air traffic control metering fixes.
The difficulty caused by this approach is found in the pragmatic necessity for a flight to reach a destination, whether it be an interim waypoint or a final destination, at a predetermined time. Requirements for such time constraints are seen in the need to arrive at an oceanic gateway within a specified time window, or the need to arrive at an airport at a specified time in order to be allowed access to the terminal. Many other requirements for time constrained navigation can be found in both the military and commercial arenas.
Any attempt to solve the problems presented by these time constraints will inherently involve the current estimated time of flight, the time of flight required by the time constraint or required time of arrival (RTA), the distance of the flight, and the speed of the aircraft. These parameters, along with those that were used to develop the original estimate of the flight time, provide the inputs that can allow for the adjustment of the flight to achieve the time constraint. In problems of this nature, the standard engineering practice has been to utilize feedback control systems to provide the solution. This method allows the system developers to approximate the activities of the system by the use of modeling techniques, and to introduce corrections through feedback to achieve desired goals.
Prior art methods and apparatus used in solving this problem did not utilize either the performance optimizer or feedback of the speed schedule into the flight profile predictions. This is illustrated in the approach taken by A. P. Palmeri in U.S. Pat. No. 4,774,670. This invention used the estimated time of arrival produced by the profile predictions to compute a time error. This time error was then applied to the cruise segment of the flight profile to obtain a speed error percentage. The speeds used in the cruise segment were then adjusted by this error percentage to obtain a new speed schedule.
The Palmeri invention was limited in a number of ways. First and foremost, the new speed schedule was not fed back into the flight profile predictor. The flight profile predictor would continue to display fuel predictions based upon the original speed schedule, not the newly generated one. Secondly, the speed schedule was only adjusted during the cruise portion of the flight. Short flights may require adjustments to the climb and descent portions to enable the system to meet the required time of arrival. Third, this approach did not allow for multiple time constraints, which are often used in transoceanic flights. By treating the cruise segment as a whole, and not the individual legs that make it up, it is not possible to adjust these legs individually. Along these same lines, constant speed segments, which are also often used in transoceanic flights, would not be permitted.
Another prior art solution for this problem has been to vary the parameters used to define the original optimization in such a way that the desired time of arrival is achieved. See, for example, U.S. Pat. No. 4,760,530, invented by S. P. Liden and assigned to the assignee of the present invention, and M. K. DeJonge in U.S. Pat. No. 5,121,325, which utilize a form of variable cost index prediction. This serves to dilute the optimal character of the resultant flight, since the parameters used to determine the optimum cost are no longer those being used to achieve the desired destination time. The flight plan that results from these new parameters is suboptimal, when considered in the light of the original flight plan.
Additional difficulties occur when this approach is implemented. It is not always clear which parameters should be selected and changed so that the resultant flight path achieves its desired time constraint. In fact, the choice of parameters to vary is arbitrary. Further, the exact result of a variation of a selected parameter is not predictable with any degree of assuredness. Instead, such gross measures as the direction of the "correction" is often in question and the effect is seldom monotonic; in fact, the magnitude of the correction can vary throughout the computation cycle. This lack of essential monotonicity leads to extended computations and often results in limit cycles that require testing and termination control.
The introduction of more than one time constraint in practical flight plans introduces still another difficulty. None of the current methods that approach the time-constrained navigation problem by varying the parameters that feed the cost function address this situation. To do so would defeat the nature of the optimized flight even further, and would increase the complexity by requiring optimization calculations to be performed over flight segments. This approach would clearly be non-optimal, and the validity of the optimization computations would be placed in question.
Alternative solutions, including the approach taken by Palmeri, have been proposed that treat the problem from the point of the global speed ratios. These solutions have used a ratio developed from each speed in the cruise segment to achieve the single objective presented by the specified arrival time. No consideration is given to the very pragmatic inclusion of constant speed segments, or to passenger comfort, or to the usual requirement for including several time constraints in a single flight plan. The time variation for a single flight plan leg is individually computed, and, since it is independent of the preceding and following legs, the speed match can easily be seen to be neglected. It is this speed match that will cause the passengers in an airline that does not perform it well to choose another form of travel. Additionally, the approaches noted have not addressed the computational difficulties associated with their solutions, involving oscillation of the solution, without the inclusion of complex mechanisms for controlling these limit cycles.
These problems suggest a search for an approach that will alleviate the calculation complexity, yield monotonic closure on the desired time constraint, provide a smooth speed variation, and still remain close to the original optimized flight path. The present invention is such an approach. It provides time constraints for an arbitrary number of legs of the flight plan, whether designated as constant speed or variable speed, without modifying the input parameters determining the optimal cost of flight. A single speed adjustment factor is applied over a given time constraint sub-path, which may be comprised of a combination of constant speed and variable speed legs, and provides for rapid settling to a required time of arrival without significantly impacting the processor requirements of an existing flight management system.